Compliant and crosslinkable thermal interface materials

ABSTRACT

Described is a compliant and crosslinkable thermal interface material of at least one silicone resin mixture, at least one wetting enhancer and at least one thermally conductive filler, and a method of making and using same; as well as a method of improving thermal conductivity of polymer and resin systems.

This application is a divisional of allowed application Ser. No.09/774,466, filed Jan. 30, 2001 now U.S. Pat. No. 6,605,238.

This application claims the benefit of U.S. application Ser. No.09/398,988 incorporated herein by reference in its entirety.

BACKGROUND

As electronic devices become smaller and operate at higher speeds,energy emitted in the form of heat increases dramatically. A popularpractice in the industry is to use thermal grease, or grease-likematerials, alone or on a carrier in such devices to transfer the excessheat dissipated across physical interfaces. Most common types of thermalinterface materials are thermal greases, phase change materials, andelastomer tapes. Thermal greases or phase change materials have lowerthermal resistance than elastomer tape because of the ability to bespread in very thin layers and provide intimate contact between adjacentsurfaces. Typical thermal impedance values range between 0.6-1.6° C.cm²/w.

A serious drawback of thermal grease is that thermal performancedeteriorates significantly after thermal cycling, such as from −65° C.to 150° C., or after power cycling when used in VLSI chips. It has beenalso found that the performance of these materials deteriorates whenlarge deviations from surface planarity causes gaps to form between themating surfaces in the electronic devices or when large gaps betweenmating surfaces are present for other reasons, such as manufacturingtolerances, etc. When the heat transferability of these materials breaksdown, the performance of the electronic device in which they are used isadversely affected. The present invention provides a thermal interfacematerial that is particularly suitable for use as an interface materialin electronic devices.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a compliant andcrosslinkable material which comprises a silicone resin mixture, such asa mixture of vinyl silicone, vinyl Q resin, hydride functional siloxaneand platinum-vinylsiloxane, a wetting enhancer and at least onethermally conductive filler. The compliant thermally conductive materialhas the capability of enhancing heat dissipation in high powersemiconductor devices and maintains stable thermal performance. It isnot subject to interfacial delamination or phase separation duringthermal-mechanical stresses or fluctuating power cycling of theelectronic devices in which it is used.

The compliant and crosslinkable thermal interface material may beformulated by mixing the components together to produce a paste whichmay be applied by dispensing methods to any particular surface and curedat room temperature or elevated temperature. It can be also formulatedas a highly complaint, cured, tacky elastomeric film or sheet for otherinterface applications where it can be preapplied, for example on heatsinks, or in any other interface situations.

The filler to be incorporated advantageously comprises at least onethermally conductive filler, such as silver, copper, aluminum, andalloys thereof; boron nitride, aluminum nitride, silver coated copper,silver coated aluminum and carbon fiber. It may be also additionallyuseful to incorporate antioxidants to reduce oxidation of the resins,wetability enhancing agents to promote wetting of surfaces, curingaccelerators, such as would allow curing at room temperature, viscosityreducing agents to enhance dispersability and crosslinking aids. It isalso often desirable to include substantially spherical particles offiller to limit the compressibility of the compliant material ininterface applications, i.e. to limit or control the thickness.

It has been also found that thermal conductivity of resin systems, suchas a combination of filler and the combined silicone resin mixturediscussed above, can be especially improved by incorporating carbonmicro fibers, with other fillers, into the system.

DETAILED DESCRIPTION

A compliant and crosslinkable thermal interface material is formulatedby combining a silicone resin mixture, such as a mixture of vinylsilicone, vinyl Q resin, hydride functional siloxane andplatinum-vinylsiloxane, an organo-titanate wetting enhancer and at leastone thermally conductive filler. More than one silicone resin mixturemay be combined to produce a compliant and crosslinkable interfacematerial. Silicone resin containing interface materials, withappropriate thermal fillers, exhibit a thermal capability of less than0.5 cm²° c./w. Unlike thermal grease, thermal performance of thematerial will not degrade after thermal cycling or flow cycling in ICdevices because liquid silicone resins will cross link to form a softgel upon heat activation. Moreover, when applied as an interfacematerial it will not be “squeezed out” as thermal grease does in use andwill not display interfacial delamination during thermal cycling. Thenew material can be provided as a dispensable liquid paste to be appliedby dispensing methods and then cured as desired. It can also be providedas a highly compliant, cured, elastomer film or sheet forpre-application on interface surfaces, such as heat sinks.Advantageously, fillers with a thermal conductivity of greater thanabout 0.2, and preferably at least about 0.4, w/m° C. will be used.Optimally, it is desired to have a filler of not less than about 1 w/m°C. thermal conductivity. The compliant thermally conductive materialenhances thermal dissipation of high power semiconductor devices. Thepaste may be formulated as a mixture of functional silicone resins andthermal fillers.

As used herein, the term “compliant” encompasses the property of amaterial that is yielding and formable at room temperature, as opposedto solid and unyielding at room temperature. As used herein, the term“crosslinkable” refers to those materials or compounds that are not yetcrosslinked.

A vinyl Q resin is an activated cure specialty silicone rubber havingthe following base polymer structure:

Vinyl Q resins are also clear reinforcing additives for addition cureelastomers. Examples of vinyl Q resin dispersions that have at least 20%Q-resin are VQM-135 (DMS-V41 Base), VQM-146 (DMS-V46 Base), and VQX-221(50% in xylene Base).

For example, a contemplated silicone resin mixture could be formed asfollows:

Component % by weight Note/Function Vinyl silicone 75 (70-97 range)Vinyl terminated siloxane Vinyl Q Resin 20 (0-25 range) Reinforcingadditive Hydride functional  5 (3-10 range) Crosslinker siloxanePlatinum-vinylsiloxane 20-200 ppm Catalyst

The resin mixture can be cured at either at room temperature or elevatedtemperature to form a compliant elastomer. The reaction is viahydrosilylation (addition cure) of vinyl functional siloxanes by hydridefunctional siloxanes in the presence of a catalyst, such as platinumcomplexes or nickel complexes. Preferred platinum catalysts areSIP6830.0, SIP6832.0, and platinum-vinylsiloxane.

Contemplated examples of vinyl silicone include vinyl terminatedpolydimethyl siloxanes that have a molecular weight of about 10000 to50000. Contemplated examples of hydride functional siloxane includemethylhydrosiloxane-dimethylsiloxane copolymers that have a molecularweight about 500 to 5000. Physical properties can be varied from a verysoft gel material at a very low crosslink density to tough elastomernetwork of higher crosslink density.

Thermal filler particles to be dispersed in the resin mixture shouldadvantageously have a high thermal conductivity. Suitable fillermaterials include silver, copper, aluminum, and alloys thereof; boronnitride, aluminum spheres, aluminum nitride, silver coated copper,silver coated aluminum and carbon fibers. Combinations of boron nitrideand silver or boron nitride and silver/copper also provide enhancedthermal conductivity. Boron nitride in amounts of at least 20 wt. %,aluminum spheres in amounts of at least 70 wt. %, and silver in amountsof at least about 60 wt. % are particularly useful.

Of special efficacy is a filler comprising a particular form of carbonfiber referred to as “vapor grown carbon fiber” (VGCF) such as isavailable from Applied Sciences, Inc., Cedarville, Ohio. VGCF, or“carbon micro fibers”, are a highly graphized type by heat treatment(thermal conductivity =1900 w/m° C.). Addition of about 0.5 wt. % carbonmicro fibers provides significantly increased thermal conductivity. Suchfibers are available in varying lengths and diameters; namely, 1 mm totens of centimeters in length and from under 0.1 to over 100 μm indiameter. One useful form has a diameter of not greater than about 1 μmand a length of about 50 to 100 μm, and possess a thermal conductivityof about two or three times greater than with other common carbon fibershaving diameters greater than 5 μm.

It is difficult to incorporate large amounts of VGCF in resin systemssuch as the silicone resin mixture discussed above. When carbon microfibers, e.g. (about 1 μm, or less, are added to the resins they do notmix well because the need to incorporate a large amount of fiberrelative to the amount of the resins for beneficial improvement thermalconductivity. However, we have found that relative large amounts ofcarbon micro fibers can be added to resin systems that have relativelylarge amounts of other fillers. A greater amount of carbon micro fiberscan be added to the resin when added with other fibers than can be addedalone to the polymer, thus providing a greater benefit with respect toimproving thermal conductivity of the thermal interface material.Desirably, the ratio of carbon micro fibers to polymer is in the rangeof 0.05 to 0.50 by weight.

It is also advantageous to incorporate substantially spherical fillerparticles to maximize packing density. Additionally, substantiallyspherical shapes or the like will also provide some control of thethickness during compaction. Dispersion of filler particles can befacilitated by addition of functional organo metallic coupling agents orwetting agents, such as organosilane, organotitanate, organozirconium,etc. Typical particle sizes useful for fillers in the resin material maybe in the range of about 1-20 μm with a maximum of about 100 μm.Antioxidants may be added to inhibit oxidation and thermal degradationof the resin mixture gel. Typical useful antioxidants include Irganoz1076, a phenol type of Irganox 565, an amine type, (at 0.01% to about 1wt. %), available from Ciba Giegy of Hawthorne, N.Y.

To illustrate the invention, a number of examples were prepared bymixing the components described in Examples A through F below. Theexamples shown include one or more of the optional additions, e.g.,antioxidant, or wetability enhancer. The amounts of such additions mayvary but, generally, they may be usefully present in the followingapproximate amounts (in wt. %): filler up to 95% of total (filler plusresins); wetability enhancer 0.1 to 5% (of total); adhesion promoters0.01 to 1% (of total) and antioxidant 0.01 to 1% (of total). It shouldbe noted the addition at least about 0.5% carbon fiber significantlyincreases thermal conductivity. The examples also show variousphysico-chemical measurements for the contemplated mixtures.

Thermal Paste Examples Component A B C D E F Silicone resin 25 20 24.819.8 16 12 mixture Epoxy silane 0.1 0.1 0.1 0.1 Silanol 0.1 0.1 0.1 0.1terminated vinylsiloxane Organo-titanate 3.8 3.8 Aluminum 75 80 75 80 8084 spheres Viscosity 80 275 78 280 85 180 (Pa · s) Adhesive 100 45 250180 160 250 strength (psi) Modulus at 20 35 22 30 14 28 25 C. (Mpa)Thermal 0.44 0.55 0.40 0.50 0.28 0.25 Impedance (cm²C/w) Thermal 1.8 1.51.8 1.5 3.0 3.2 Conductivity (w/mC)

Epoxy silane and silanol terminated vinyl siloxane are adhesionpromoting additives. Organo-titanate acts a wetting enhancer to reducepaste viscosity and to increase filler loading. The organo-titanate usedwas isopropyl triisostearyl titanate. General structure oforgano-titanate is RO—Ti(OXRY) where RO is a hydrolyzable group, X and Yare binder functional groups. Aluminum spheres have particle sizes inthe range of 1-20 um.

Thus, specific embodiments and applications of compliant andcrosslinkable thermal interface materials have been disclosed. It shouldbe apparent, however, to those skilled in the art that many moremodifications besides those already described are possible withoutdeparting from the inventive concepts herein. The inventive subjectmatter, therefore, is not to be restricted except in the spirit of theappended claims. Moreover, in interpreting both the specification andthe claims, all terms should be interpreted in the broadest possiblemanner consistent with the context. In particular, the terms “comprises”and “comprising” should be interpreted as referring to elements,components, or steps in a non-exclusive manner, indicating that thereferenced elements, components, or steps may be present, or utilized,or combined with other elements components, or steps that are notexpressly referenced.

We claim:
 1. An electronic device comprising a compliant andcrosslinkable material, wherein the material comprises at least onesilicone resin mixture and at least one thermally conductive filler, andwherein the at least one silicone resin mixture comprises a vinyl Qresin.
 2. The electronic device of claim 1, wherein the material furthercomprises at least one wetting enhancer.
 3. The electronic device ofclaim 1, wherein the at least one silicone resin mixture comprises avinyl terminated siloxane, a crosslinker and a catalyst.
 4. Theelectronic device of claim 3, wherein the vinyl terminated siloxane isvinyl silicone.
 5. The electronic device of claim 3, wherein thecrosslinker comprises a hydride functional siloxane.
 6. The electronicdevice of claim 3, wherein the catalyst comprises a platinum complex. 7.The electronic device of claim 6, wherein the platinum complex is aplatinum-vinylsiloxane compound.
 8. The electronic device of claim 2,wherein the wetting enhancer comprises an organo-titanite compound. 9.The electronic device of claim 1, wherein the filler comprises at leastone of silver, copper, aluminum, and alloys thereof; boron nitride,aluminum nitride, aluminum spheres, silver coated copper, silver coatedaluminum, carbon fibers and mixtures thereof.
 10. The electronic deviceof claim 9, wherein the material further comprises carbon micro fibersin addition to at least one other filler.
 11. The electronic device ofclaim 1, wherein the filler comprises carbon micro fibers and at leastone other filler comprising silver, copper, aluminum, and alloysthereof; boron nitride, aluminum nitride, silver coated copper, silvercoated aluminum, and carbon fibers; and mixtures thereof.
 12. Theelectronic device of claim 1, wherein at least part of the fillercomprises substantially spherical particles.
 13. The electronic deviceof claim 1, wherein the material further comprising an antioxidant. 14.The electronic device of claim 13, wherein the antioxidant is present inan amount of 0.01 to 1 weight percent.
 15. A dispensable liquid pastecomprising a compliant and crosslinkable material, wherein the materialcomprises at least one silicone resin mixture and at least one thermallyconductive filler, and wherein the at least one silicone resin mixturecomprises a vinyl Q resin.
 16. The dispensible liquid paste of claim 15,further comprising at least one wetting enhancer.
 17. The dispensibleliquid paste of claim 15, wherein the at least one silicone resinmixture comprises about 70 to about 97 percent by weight of avinyl-terminated siloxane, about 3 to about 10 percent by weight of acrosslinker and about 20 to about 200 ppm of a catalyst.
 18. Thedispensible liquid paste of claim 17, wherein the at least one siliconeresin mixture comprises about 75 percent by weight of a vinyl-terminatedsiloxane, about 5 percent by weight of a crosslinker and about 20 toabout 200 ppm of a catalyst.
 19. The dispensible liquid paste of claim18, wherein the vinyl-terminated siloxane comprises vinyl silicone. 20.The dispensible liquid paste of claim 18, wherein the crosslinkercomprises hydride functional siloxane.
 21. The dispensible liquid pasteof claim 18, wherein the catalyst comprises platinum-vinylsiloxane. 22.The dispensible liquid paste of claim 15, wherein the at least onesilicone resin mixture is curable at room temperature.